Scanning electron micrograph of the surface of garden soil. The picture, covering an area of 0.75mm x 0.5mm, shows small clods of soil, densely populated by unicellular algae. Identification to species level in this picture is not possible, but different cell forms can be distinguished. Most of the cells are spherical, corresponding to members of the Chlorophyta (green algae) such as Chlorella. Others are shaped like elongated boxes with rounded tips, or with straight rectangular edges. These are pennate diatoms. In the centre of the picture, the curving tube (pink) is the body of a nematode worm. Elsewhere, fragments of material of indefinite shape are remnants of vegetation as yet incompletely decomposed by - invisible at this magnification - millions of bacteria. A fertile soil is full of life; each component contributes to that fertility, on which as humans we depend for our food.